This invention relates generally to organic electroluminescent devices with organic films between anodic and cathodic electrodes, and more particularly to such devices and methods for their assembly using the condensation of various silicon moieties.
Organic electroluminescent devices have been known, in various degrees of sophistication, since the early 1970's. Throughout their development and consistent with their function and mode of operation, they can be described generally by way of their physical construction. Such devices are characterized generally by two electrodes which are separated by a series of layered organic films that emit light when an electric potential is applied across the two electrodes. A typical device can consist, in sequence, of an anode, an organic hole injection layer, an organic hole transport layer, an organic electron transport layer, and a cathode. Holes are generated at a transparent electrode, such as one constructed of indium-tin-oxide, and transported through a hole-injecting or hole-transporting layer to an interface with an electron-transporting or electron-injecting layer which transports electrons from a metal electrode. An emissive layer can also be incorporated at the interface between the hole-transporting layer and the electron-transporting layer to improve emission efficiency and to modify the color of the emitted light.
Significant progress has been made in the design and construction of polymer- and molecule-based electroluminescent devices, for light-emitting diodes, displays and the like. Other structures have been explored and include the designated “DH” structure which does not include the hole injection layer, the “SH-A” structure which does not include the hole injection layer or the electron transport layer, and the “SH-B” structure which does not include the hole injection layer or the hole transport layer. See, U.S. Pat. No. 5,457,357 and in particular col. 1 thereof, which is incorporated herein by reference in its entirety.
The search for an efficient, effective electroluminescent device and/or method for its production has been an ongoing concern. Several approaches have been used with certain success. However, the prior art has associated with it a number of significant problems and deficiencies. Most are related to the devices and the methods by which they are constructed, and result from the polymeric and/or molecular components and assembly techniques used therewith.
The fabrication of polymer-based electroluminescent devices employs spin coating techniques to apply the layers used for the device. This approach is limited by the inherently poor control of the layer thickness in polymer spin coating, diffusion between the layers, pinholes in the layers, and inability to produce thin layers which leads to poor light collection efficiency and the necessity of high D.C. driving voltages. The types of useful polymers, typically poly(phenylenevinylenes), are greatly limited and most are environmentally unstable over prolonged use periods.
The molecule-based approach uses vapor deposition techniques to put down thin films of volatile molecules. It offers the potential of a wide choice of possible building blocks, for tailoring emissive and other characteristics, and reasonably precise layer thickness control. Impressive advances have recently been achieved in molecular building blocks—especially in electron transporters and emitters, layer structure design (three versus two layers), and light collection/transmission structures (microcavities).
Nevertheless, further advances must be made before these devices are optimum. Component layers which are thinner than achievable by organic vapor deposition techniques would allow lower DC driving voltages and better light transmission collection characteristics. Many of the desirable component molecules are nonvolatile or poorly volatile, with the latter requiring expensive, high vacuum or MBE growth equipment. Such line-of-site growth techniques also have limitation in terms of conformal coverage. Furthermore, many of the desirable molecular components do not form smooth, pinhole-free, transparent films under these conditions nor do they form epitaxial/quasiepitaxial multilayers having abrupt interfaces. Finally, the mechanical stability of molecule-based films can be problematic, especially for large-area applications or on flexible backings.
In order to realize high resolution information displays, the microfabrication of light-emitting diodes and pixel arrays is required. So far, several approaches have been employed. One such approach involves patterning the anode and/or cathode of the device structure, the other patterning the emitting materials. The bottom ITO electrode is commonly patterned using a combination of standard photolithography and wet chemical etching, while the top electrode is typically defined by deposition through a shadow mask. Generally, there are disadvantages associated with each of these procedures, such as the difficulty of precise shadow mask alignment for multiple deposition, and frequent mask replacement or cleaning for high dimensional control of the underlying display panel. Alternatively, the cathode and/or anode could be altered using laser ablation. However, with patterned substrates, vacuum deposition of the organic materials does not efficiently coat the steep edges and sharp corners of the anode, thereby resulting in poor coverage which can lead to cathode-to-anode short circuits.